A metallic glass is a metastable alloy that does not have any long-range order, in contrast to normal crystalline materials. Its structure is amorphous and is similar to that of a liquid. Several conditions must be met in order to obtain the amorphous state during cooling. For instance, the nucleation and nucleus growth must be suppressed in order to freeze the structure of the liquid. In order to realize this, the metallic melt must be cooled very quickly, for example, through contact with the surface of a heat sink that conducts heat very well. The quality of the thermal contact and the thickness and heat conductivity of the liquid layer determine the cooling rate.
A known and very widespread method of casting metals as well as solid metallic glasses is casting into cold ingot molds. The melt is thereby forced into the ingot mold by various methods and sets there in the shape predetermined by the ingot mold.
In order to obtain high cooling rates in the case of metallic glasses, the ingot mold is produced from a material that conducts heat well. The casting operation thereby takes place very quickly. Firstly, the metal is melted in a crucible, and subsequently the melt is forced into the mold by gas pressure or centrifugal force.
The surface of the mold must be very clean in order to ensure a good thermal contact between the metallic melt and the ingot mold, advantageously made of copper. This can be easily realized by mechanical cleaning and pickling. Moreover, the melt should wet the mold well. The wetting depends essentially on the viscosity and interfacial surface tension of the melt with respect to the copper ingot mold and with respect to the ambient atmosphere. The viscosity depends very much on the temperature. It decreases exponentially with rising temperature, while the interfacial surface tensions decrease linearly with rising temperature. Low values for viscosity and interfacial surface tension, such as are desirable for a good wetting and for a good filling of the mold, can be set in principle by a high temperature. However, an increase in temperature also results in a higher quantity of heat to be removed, which leads to a lower cooling speed and therefore is not desirable. Overheating the melt when casting crystalline alloys leads to a good filling of the mold, but overheating should be avoided when casting metallic glasses in order to be able to freeze the amorphous state.
It is furthermore known that contaminations of the melt with oxygen interfere with the producibility of metallic glasses and impair their properties. This effect is explained by heterogeneous nucleation on oxide particles in the melt. A method for electrochemical cleaning of the melt before casting through a current flow between a slag floating on the melt and the metal melt was described by S. Bossuyt et al., Mater. Sci. Eng. A 375-377 (2004) 240-243.